The invention relates to methods for preparing polymer films. Specifically, the invention relates to methods of preparing bioriented polyethylene films having high water vapor transmission rates (WVTR).
Generally, in the preparation of a film from granular or pelleted polymer resin, the polymer is first extruded to provide a stream of polymer melt, and then the extruded polymer is subjected to the film-making process. Film-making typically involves a number of discrete procedural stages, including melt film formation, quenching, and windup. For a general description of these and other processes associated with film-making, see K R Osborn and W A Jenkins, Plastic Films: Technology and Packaging Applications, Technomic Publishing Co., Inc., Lancaster, Pa. (1992).
An optional part of the film-making process is a procedure known as xe2x80x9corientation.xe2x80x9d The xe2x80x9corientationxe2x80x9d of a polymer is a reference to its molecular organization, i.e., the orientation of molecules relative to each other. Similarly, the process of xe2x80x9corientationxe2x80x9d is the process by which directionality (orientation) is imposed upon the polymeric arrangements in the film. The process of orientation is employed to impart desirable properties to films, including making cast films tougher (higher tensile properties). Depending on whether the film is made by casting as a flat film or by blowing as a tubular film, the orientation process requires substantially different procedures. This is related to the different physical characteristics possessed by films made by the two conventional film-making processes: casting and blowing. Generally, blown films tend to have greater stiffness, toughness and barrier properties. By contrast, cast films usually have the advantages of greater film clarity and uniformity of thickness and flatness, generally permitting use of a wider range of polymers and producing a higher quality film.
Orientation is accomplished by heating a polymer to a temperature at or above its glass-transition temperature (Tg) but below its crystalline melting point (Tm), and then stretching the film quickly. On cooling, the molecular alignment imposed by the stretching competes favorably with crystallization and the drawn polymer molecules condense into a crystalline network with crystalline domains (crystallites) aligned in the direction of the drawing force. As a general rule, the degree of orientation is proportional to the amount of stretch, and inversely related to the temperature at which the stretching is performed. For example, if a base material is stretched to twice its original length (2:1) at a higher temperature, the orientation in the resulting film will tend to be less than that in another film stretched 2:1 but at a lower temperature. Moreover, higher orientation also generally correlates with a higher modulus, i.e., measurably higher stiffness and strength. Further, as a general rule, higher orientation correlates with lower WVTR values for films.
Previously, high WVTR values have been difficult to achieve with polyolefin films. Typically, film production methods aim to lower WVTR values for polyolefin films that have inherently low WVTR values compared to traditional wrapping materials such as cellulose films or paper.
Accordingly, it is one of the purposes of this invention, among others, to produce bioriented polyethylene films having high WVTR values, by providing an economical and relatively uncomplicated method of making polyethylene films that imparts superior characteristics to the films, without requirement for chemical additives such as cross-linking agents, and without requirement for supplemental processing steps such as irradiation of the film.
It has now been discovered that these and other purposes can be achieved by the present invention, which provides methods of producing polyethylene films having high water vapor transmission rates.
The methods provide for casting and then biaxially orienting a polyethylene sheet to obtain a film having a desired WVTR. The polyethylene sheet comprises a base layer comprising polyethylene and a cavitating agent, and at least one layer of a WVTR-controlling material coextensively adherent to a side of the base layer. The polyethylene sheet is biaxially oriented whereby there is provided a bioriented polyethylene film having the desired WVTR and whereby the base layer has a porous microstructure and a WVTR substantially higher than the desired WVTR.
Preferably, the polyethylene of the base layer is a medium density polyethylene (MDPE) or a high density polyethylene (HDPE). Further, it is preferable that the polyethylene in the base layer is provided in an amount sufficient to yield a base layer in the film having a thickness of from about 0.5 mil to about 2.0 mil (1 mil=0.001 inch=100 gauge), preferably about 0.85 mil (85 gauge) to about 1.10 mil (110 gauge).
In addition, it is preferable that the WVTR-controlling material is a MDPE or a HDPE, however, the WVTR-controlling material should not have a density greater than that of the polyethylene in the base layer. Further, it is preferable that the WVTR-controlling material is provided in an amount sufficient to yield a WVTR-controlling layer in the film having a thickness of from about 0.03 mil (3 gauge) to about 0.15 mil (15 gauge).
A preferred method of the present invention provides for producing a film from a polyethylene sheet having a three layer structure. In particular, a sheet having first and second layers of a WVTR-controlling material coextensively adherent to first and second sides of the base layer is provided for casting and then biaxial orienting.
Another preferred method of the present invention provides for producing a film from a polyethylene sheet having a five layer structure. In particular, the sheet provided for casting comprises a base layer interposed between two tie layers wherein the first tie layer is interposed between one side of the base layer and a first WVTR-controlling layer whereby the first tie layer is coextensively adherent to the base layer and the WVTR-controlling layer, and wherein the second tie layer is interposed between the other side of the base layer and a second WVTR-controlling layer whereby the second tie layer is coextensively adherent to the base layer and the WVTR-controlling layer.
Another method of the present invention provides for producing a film from a sheet having a structure wherein at least one tie layer is interposed between the base layer and a WVTR-controlling layer and the tie layer is coextensively adherent to the base layer and the WVTR-controlling layer. The WVTR-controlling layer comprises a WVTR-controlling material of preferably HDPE or MDPE. However, an alternative method includes provision for a WVTR-controlling layer comprising a WVTR-controlling material of an ethylene-propylene copolymer or an ethylene-propylene-butylene terpolymer wherein the tie layer comprises low density polyethylene (LDPE) or MDPE.
The present invention provides methods of producing polyethylene films having high WVTR values, opacity, high stiffness and resistance to humidity. The films also have excellent deadfold characteristics which make them well suited for packaging of foods in bag-in-box operations conducted on vertical, form, fill and seal (VFFS) machinery. These properties make these films an excellent alternative to paper or cellophane in applications where high WVTR and insensitivity of film to moisture are required.
These and other advantages of the present invention will be appreciated from the detailed description and examples which are set forth herein. The detailed description and examples enhance the understanding of the invention, but are not intended to limit the scope of the invention.